This invention relates to an improved process for the preparation of aminomethanols.
N-(aminomethyl)-.alpha.,.beta.-ethylenically unsaturated carboxamides may be prepared by carrying out an acid-catalyzed condensation reaction (Mannich reaction) of acrylamide, a secondary amine and a lower aldehyde, usually formaldehyde. See, for example, U.S. Pat. No. 4,166,828. Said carboxamides may be polymerized to form a cationic poly N-(aminomethyl)acrylamide which is useful as a flocculant in water and sewage treatment processes. The Mannich reaction of acrylamide is carried out either by simultaneously reacting the acrylamide, the amine and a lower aldehyde, or by prereacting the amine with the aldehyde. Both of the above processes are known in the art. See U.S. Pat. Nos 3,539,535 and 4,166,828.
In the above processes, unreacted aldehydes can react directly with the acrylamide, forming an N-methylol amide represented by the structure: ##STR1## wherein R.sub.1 is hydrogen or an alkyl group having 1 or 2 carbon atoms. Under some conditions this by-product forms a methylene-bis-amide, an undesired cross-linking agent represented by the general structure: ##STR2## wherein R.sub.1 is as described hereinbefore. Since cross-linking in the poly N-(aminomethyl)acrylamide decreases its solubility and hence its value as a flocculant, it is desirable to prepare the Mannich monomer free of cross-linking impurities.
Thus, careful control of the acid-catalyzed Mannich reaction of acrylamide monomer is desirable to minimize the formation of the undesired N-methylol amide and methylene-bis-amide. One method of controlling the reaction is by reacting the amine with the aldehyde to form an aminomethanol under basic conditions and then acidifying this reagent prior to introducing the acrylamide into the mixture. The formation of the undesired by-products is reduced in this manner, but unreacted aldehyde in the aminomethanol can still react with the acrylamide monomer to form cross-linking impurities. Thus, further improvement in the process can be achieved by removing the unreacted aldehyde from the aminomethanol prior to introducing of the acrylamide into the mixture. However, previously known methods of treating the aminomethanol to remove the unreacted aldehyde, such as by employing excess amine in the reaction or by heating the aminomethanol, leads to considerable degradation of the aminomethanol into useless and undesired by-products.
In view of the deficiencies of conventional processes of forming N-(aminomethyl)acrylamides, it is highly desirable to develop a process whereby aminomethanols are prepared in stable form with reduced amounts of unreacted aldehydes.